The SKP1/Cullin/F-box (SCF) class of E3 Ubiquitin Ligases is an evolutionarily conserved family of enzymes that regulate key cellular processes including the cell cycle, membrane trafficking, and signaling. The SCF-E3 is composed of four core components - Rbx1, Cullin1, SKP1, and a F-box protein. F-box proteins are the subunits that recruit substrate proteins to be poly-ubiquitinated by the SCF-E3, and they can also be directly poly-ubiquitinated. SCF-E3 dependent polyubiquitin serves as a signal for targeting to and degradation by the 26S-proteasome. Every eukaryote expresses a repertoire of F-box proteins, the majority of which have different affinities for different substrate proteins. Thus, F-box proteins are critical since they dictate which specific proteins will be ubiquitinated by the SCF-E3. In addition, F-box proteins can function independently of the SCF-E3. Thus, identifying and characterizing F-box proteins is central to understanding the functions of the SCF-E3 and remodeling of the proteome as cells engage new functions. This is important because protein degradation is as important as gene transcription in defining a cell's proteome. Here, we will identify essential SCF-E3 dependent F-box proteins, determine why they are essential, and identify the proteins that are their ubiquitination substrates. These studies will reveal how ubiquitin supports growth and virulence of a major human parasite. In addition, these findings will be relevant to related pathogens given the conservation of the SCF- E3 and many F-box proteins between Toxoplasma and other apicomplexan parasites.